A Bureau statement issued just before 5am reads that a trough along the NSW coast was causing significant rainfall along the coastal reaches.

A trough lies along the NSW coast, with a low pressure system forecast to develop along the trough off the north coast during the course of Wednesday. This system is directing persistent and widespread rain and isolated thunderstorms over the southeast.

The Bureau warns of continuing heavy rainfall following Tuesday night's storms with the potential for flash flooding.

Thunderstorms are expected and may locally enhance rainfall.

Locations which may be affected include Nowra, Batemans Bay, Eden, Pambula, Merimbula, Tathra Bega, and north to Moruya Heads and Nerriga.

The Bureau said overnight rainfall had shattered the forecast with Black Range hit the hardest, swamped by 120mm through to 4am on Wednesday.

Other totals from 9am Tuesday to 9am Wednesday include

- 155 mm at Black Range

- 120mm Bega R at Kanoona

- 119mm Kameruka Estate

- 115mm Bega AWS

Earlier recordings at 4am included

- 112mm Angledale

- 95 mm at Lake Tabourie

- 68 mm at Burrill Lake

- 64 mm at Perisher Valley

- 64 mm at Moruya

- 63 mm at Merimbula Airport.

The State Emergency Service advises that people should:

* Don't drive, ride or walk through flood water.

* Keep clear of creeks and storm drains.

* If you are trapped by flash flooding, seek refuge in the highest available place and ring 000 if you need rescue.

* Be aware that run-off from rainfall in fire affected areas may behave differently and be more rapid. It may also contain debris such as ash, soil, trees and rocks.

* Stay vigilant and monitor conditions. Note that the landscape may have changed following bushfires.

* For emergency help in floods and storms, ring your local SES Unit on 132 500.